The present invention relates to electrical heating devices for heating liquids, particularly those employing thermistors with positive temperature coefficient of resistance (PTC) as heating elements.
Positive temperature coefficient (PTC) heating elements, such as thermistors, are used in electrical heating devices, such as electrical radiators, electrical heating fans, and air conditioner heaters. They have an advantage over electric wire heaters in that they are self-regulating as to temperature and thus are not subject to overheating even in response to abnormal electric currents. In many prior art applications employing PTC thermistor heating elements, heat is extracted from the device by air flow through the device, including the heating elements and radiating elements, such as radiating fins. PTC heating elements, however, are very sensitive to temperature variations, even those caused by air flow across them, due to the xe2x80x9cpinch effectxe2x80x9d (current displacement), which causes reduced heating efficiency and generating power and reduces their lifetime. As will be appreciated by persons skilled in the art, this problem is more severe for applications wherein the PTC device is used to heat a liquid.
U.S. Pat. Nos. 5,598,502 and 5,471,034 disclose PTC thermistor devices for heating liquids wherein the PTC heating elements are enclosed. In both these cases, the devices disclosed have thermal resistance between the PTC elements and heat transfer members that reduces their heat transfer efficiency and, thus, their power output. The former device has the additional disadvantage of not being fully immersible. The latter device employs channels for the liquid through heat transfer members, which complicates their construction and integration into liquid systems to be heated.
The present invention seeks to provide an electrical heating device for heating liquids employing positive temperature coefficient (PTC) thermistors as heating elements, which overcomes disadvantages of known art by providing a total enclosure for the PTC heating elements and low thermal resistance between the heating elements and the radiator elements. These features allow a heating device that is totally immersible with efficient internal heat transfer, high power output, and long life.
There is thus provided, in accordance with a preferred embodiment of the invention, an electrical heating device for heating liquids employing one or more positive temperature coefficient (PTC) thermistors as heating elements. These heating elements are in direct thermal and electrical contact on opposing sides, which are coated with a conductive metal such as aluminum, with electrodes to supply electrical current. These electrodes are, in turn, in direct thermal contact on their outward-facing sides with respect to the heating elements with plates which are formed of a thermally conductive and electrically insulating ceramic material such as AL2O3, and the plates are in direct thermal contact on their outward-facing sides with respect to the heating elements with heat radiation units which include cooling fins for heat transfer. The heating elements are positioned by an electrically and thermally insulating frame made of heat-resistant material which serves, together with the radiation units, to fully enclose the heating elements, thereby preventing liquid from entering the interior of the device, thus rendering it immersible.
The internal elements of the heating device are further held in place and in good thermal contact with each other by mechanical pressure and by a thermally and electrically conductive adhesive. The insulating plates are coated on their inward-facing with respect to the heating elements with a conductive metal such as aluminum to provide good thermal contact therewith. The conductive coatings on the insulating plates can serve as the electrodes for the device, or the electrodes can optionally be independent elements with a textured surface to provide good thermal contact with adjacent members of the device.